A conventional thin film compound solar cell is configured such that a surface electrode is provided on a light receiving surface of a cell main body formed by laminating a plurality of compound semiconductor layers, and such that a rear surface electrode is provided on the surface opposite to the light receiving surface of the cell main body.
The thin film compound solar cell is manufactured as follows. In process A1 shown in FIG. 24, an etching-stop layer 102, a base layer 103, an emitter layer 104, and a contact layer 105 respectively formed by compound semiconductor layers are laminated in this order on a substrate 101, so that a cell main body is formed.
In process A2 shown in FIG. 25, a protective film, such as a photoresist, is applied on the surface of the contact layer 105, and the region of the protective film, which region is patterned by exposure, is etched. The contact layer 105 is patterned by contact layer etching. The applied resist is removed after the completion of the patterning. Next, a photoresist is again applied for formation of a surface electrode, and a protective film is formed.
In process A3 shown in FIG. 26, a protective film opening section is formed by patterning the photoresist by exposure so that the protective film opening section is included in the region of the contact layer 105 formed by the preceding process. After a surface electrode is laminated, the photoresist is removed, so that a surface electrode 106 is selectively formed only in the protective film opening section. By this surface electrode forming process, the region of the surface electrode can be patterned so as to be included in the region of the contact layer formed by the preceding process.
After the completion of the patterning of the surface electrode 106, the surface electrode 106 is annealed at a temperature of about 350° C. in order to reduce the component of contact resistance between the surface electrode 106 and the contact layer 105 and to increase the adhesive force between the surface electrode 106 and the contact layer 105.
In process A4 shown in FIG. 27, the protective film is patterned by exposure so as to define a cell formation region corresponding to a predetermined shape (chip shape) of a solar cell element. A protective film opening section is formed, so that the opening section is mesa-etched. Then, the solar cell element having the predetermined shape (chip shape) is separated by mechanical means, such as dicing.
In process A5 shown in FIG. 28, a transparent resin, such as silicone resin, is applied to the side of the light receiving surface of the solar cell element, so that a transparent surface film 107 is bonded onto the transparent resin. Thereby, the thin film compound solar cell and the surface film 107 are bonded to each other via the resin, so that the surface film 107 serves as a base material of the thin film compound solar cell.
In process A6 shown in FIG. 29, a reinforcing material 108, such as glass or sapphire, is bonded via wax on the side of the light receiving surface of the solar cell element, to which side the surface film 107 is bonded.
In process A7 shown in FIG. 30, the solar cell element, to which the reinforcing material 108 is bonded, is immersed in an etchant. Since the etching is stopped at the etching-stop layer 102, only the substrate 101 can be removed so that only the cell main body is left. Thereby, the substrate 101 is separated from the compound semiconductor layers, so that the solar cell element exhibits its flexibility.
In process A8 shown in FIG. 31, an electrode material is vapor-deposited on the exposed rear surface of the compound semiconductor layer, so that a rear surface electrode 109 is formed.
In process A9 shown in FIG. 32, the wax bonding the reinforcing material 108 to the solar cell element is finally is dissolved by an organic solvent, such as acetone, so that the reinforcing material 108 is removed from the solar cell element.
The thin film compound solar cell manufactured as described above has the structure in which the surface film as the base material is bonded to the light receiving surface of the cell main body with a PN junction formed therein.
Meanwhile, the surface film is bonded on the side of the light receiving surface, and hence high transparency is required for the surface film so as to prevent the conversion efficiency of the solar cell element from being impaired. The high transparency film generally has low temperature resistance. In the conventional method for manufacturing the thin film compound solar cell, the process is performed such that, after the surface film is bonded to the solar cell element, the substrate is removed and the rear surface electrode is formed. After the formation of the rear surface electrode, it is necessary to anneal the rear surface electrode in order to reduce the component of contact resistance between the rear surface electrode and the compound semiconductor layer, and in order to increase the adhesive force between the rear surface electrode and the compound semiconductor layer. The annealing temperature is higher than the heat-resistant temperature of the surface film, and hence the rear surface electrode cannot be annealed in the state where the surface film is bonded to the solar cell element. Therefore, there is a problem that the rear surface electrode is separated from the compound semiconductor layer.
Further, when the wax used to bond the solar cell element to the reinforcing material is dissolved by the organic solvent, the resin used to bond the compound semiconductor layer to the surface film is also exposed to the organic solvent at the same time. Thus, there is a problem that, when the resin is exposed to the organic solvent and water, the exposed resin penetrates into the interface between the surface film and the resin, or into the interface between the compound semiconductor layer and the resin, so as to make the surface film liable to be separated from the compound semiconductor layer.
Further, the process for removing the substrate by the etchant is performed after a metallic ribbon for establishing electrical connection is welded to the solar cell element. As the etchant for etching the substrate, it is necessary to use hydrofluoric acid, and the like, depending on the substrate material. However, hydrofluoric acid reacts with the metallic ribbon to corrode the metallic ribbon. At the time of etching the substrate, the exposed metallic ribbon is required to be protected by being covered with an acid resistant material. This results in a problem that the number of processes is increased.
Here, a method for manufacturing the compound solar cell described in Patent Document 1 includes: forming the rear surface electrode on the cell main body; attaching a support plate on the rear surface electrode; separating the substrate from the cell main body to expose the surface of the cell main body; forming the surface electrode on the exposed surface of the cell main body; and then removing the support plate.
In the compound solar cell described in Patent Document 1, the rear surface electrode is formed first. For this reason, after the rear surface electrode is annealed, the surface film can be bonded. However, in the above-described compound solar cell described in Patent Document, the surface film as the base material is not provided, and hence the problem of separation of the surface film does not occur.